Recently, there has been a growing need for durable, long-lived, maintenance-free industrial equipmentand structures. As a material that meets this need, attention has been given to stainless steels. However, the prices of alloying elements such as nickel, molybdenum, and chromium, which are main raw materials for stainless steels, tend to rise and fluctuate. Accordingly, particular attention has been given to stainless steel clad steels as an alternative to solid stainless steels. Stainless steel clad steels provide for good corrosion resistance of solid stainless steels to be more economically utilized and are available at low and stable prices. A stainless steel clad steel is manufactured by bonding together two types of steels with different properties, i.e., a stainless steel as a cladding material and an ordinary steel as a base material. The clad steel, which has a metallurgical bond between different types of steels, provides novel properties that cannot be achieved by a single metal or alloy and, unlike a plated steel, does not have the risk of peeling of the plated layer. A stainless steel clad steel uses a smaller amount of stainless steel but provides a comparable corrosion resistance to a solid material (which refers to a material made of a stainless steel used as a cladding material throughout the thickness thereof). This is advantageous both economically and functionally.
As discussed above, stainless steel clad steels are considered significantly useful functional steels and, recently, there has been a growing need for stainless steel clad steels in various industrial fields. In particular, there is a need for stainless steel clad steels with high sea water corrosion resistance in applications where they are used in highly corrosive sea water environments, including marine structures, sea water desalination equipment, and vessels such as floating production, storage, and offloading systems (hereinafter referred to as “FPSO”).
A passivation film on a stainless steel fails more easily in the presence of chloride ions. The mode of corrosion of the passivation film is pitting corrosion or crevice corrosion. Whereas the mode of corrosion in acids such as sulfuric acid and hydrofluoric acid is general corrosion, the mode of corrosion in sea water is local corrosion. To eliminate origins of local corrosion, it is very important to consider pitting corrosion resistance.
The pitting corrosion resistance of a stainless steel is typically expressed as the pitting index: Cr (% by mass)+3.3Mo (% by mass)+16N (% by mass). A stainless steel having a higher pitting index has an excellent pitting corrosion resistance. This parameter, however, is applicable only to a solid stainless steel subjected to heat treatment to dissolve precipitates and so on, and is not directly applicable to the pitting corrosion resistance of a cladding material of a stainless steel clad steel, which is a composite material of a stainless steel and a carbon steel.
A stainless steel clad steel is generally normalized by heating to the temperature range of 850° C. to 950° C. and then air cooling to provide good mechanical properties such as high base material strength and toughness before use. A stainless steel may exhibit significantly low corrosion resistance after improper heat treatment or thermal history in a heat-affected zone during welding. One cause of the low corrosion resistance is that carbides and intermetallic compounds such as σ phase precipitate after improper heat treatment or thermal history in a heat-affected zone during welding. The thermal history of a clad steel varies with the thickness thereof. In particular, carbides and intermetallic compounds such as a phase precipitate more readily in a clad steel having a thickness of 30 mm or more because of its low cooling rate.
To prevent a decrease in the corrosion resistance of a clad steel after normalization, which is performed to provide good mechanical properties of the base material, Japanese Unexamined Patent Application Publication No. 63-125615 discloses a method that specifies the composition of the cladding material to maintain high corrosion resistance of the austenite stainless steel used as the cladding material after normalization at 850° C. to 950° C. The stainless steel clad steel provided by that method, however, has insufficient sea water corrosion resistance, i.e., a corrosion rate of about 2 g/m2·h as measured by a ferric chloride corrosion test.
Japanese Patent No. 4179133 discloses a method that specifies the conditions of solid solution heat treatment and the composition of a base carbon steel to manufacture a stainless steel clad steel pipe using a stainless steel with good sea water resistance as a cladding material and a carbon steel as a base material. Japanese Patent No. 4179133, however, requires the stainless steel used as the cladding material to be selected depending on the application of stainless steel clad steel pipes (such as marine structures) to provide the corrosion resistance, including sea water corrosion resistance, required for that application. That is, the method of adjustment disclosed in Japanese Patent No. 4179133 depends only on the composition of the stainless steel. Thus, if that method is applied to a stainless steel clad steel, it is difficult to ensure bonding interface integrity (adhesion) with improved reliability while simultaneously maintaining good properties (corrosion resistance and mechanical properties) of the base material and the cladding material. In addition, it is further difficult to apply this method to all of high-grade steels and various other steels.
It could therefore be helpful to provide a cladding material for stainless steel clad steel plates, with good sea water corrosion resistance that offers bonding interface integrity with improved reliability while simultaneously maintaining good properties (corrosion resistance and mechanical properties) of the base material and the cladding material, and also to provide a stainless steel clad steel plate including such a cladding material and a method of manufacturing such a stainless steel clad steel plate.